1. Field of the Invention
The present invention generally relates to a graphic processing technology, and more particularly, to a reconstructable geometry shadow mapping method.
2. Description of Related Art
Shadow mapping and shadow volume are two popular real time shadowing technologies for computer graphics. Shadow volume is proposed by Frank Crow in 1977, in which the geometry is employed for calculating the 3-dimensional (3-D) shape of a region occluded from a light source. The algorithm utilizes a stencil buffer to calculate whether a pixel is in a shadow or not. A main advantage of the shadow volume technology is that shadow volumes are relatively accurate with respect to the pixel. On the contrary, the accuracy of a shadow map depends on the size of the texture memory as well as how the shadows are casted. Unfortunately, the shadow volume technology demands for a lot of fill time on hardware and runs often slower than the shadow mapping technology, especially when processing a large scale complex geometrical scene.
Shadow mapping is a technology of adding shadows to 3-D computer graphics, which was proposed by Lance Williams in 1978. The algorithm has been widely used in pre-rendered scenes, as well as in real-time applications. A shadow of object is created by testing whether a testing pixel is visible from a light source (by comparing depths of the light-occluding object and the test pixel with respect to a light's view). Shadow mapping is an image space method with simplicity and flexibility. Shadow mapping is one of shadow rendering methods and often runs at a high speed. However, it suffers from aliasing errors and depth bias issues. As such, it becomes an important concern to provide an effective solution to these two drawbacks in the field of shadow rendering techniques.
Aliasing errors in shadow mapping can be categorized into two groups, perspective aliasing errors and projective aliasing errors. A perspective aliasing error often occurs when zooming into a shadow edge. A projective aliasing error often occurs when light rays are nearly parallel to the geometry surface and stretch over a range of depth. Another difficulty for most shadow mapping technologies is the depth bias issue. In order to avoid erroneous self-shadowing problems, William had proposed to add a constant depth bias to depth samples before comparing with a true surface. Unfortunately, too much bias may cause false non-shadowing (looks like a light-occluding object floating above the surface of a light receiver) and push the shadow backward too much. In fact, it is very hard to directly determine the bias value, and it is even impossible to find out an acceptable common value for every simple scenes.